Conductive traces are well known in the art and are typically produced with very small, thin copper pathways as the conductive material. One problem associated with copper traces is that copper is subject to corrosion from various sources. Copper forms compounds with oxidation states +1 (cuprous) and +2 (cupric). Although copper does not react with water, it does react with atmospheric oxygen forming a layer of brown-black copper oxide. Oxidation of the surface of the copper forms a green layer of verdigris (copper carbonate) that protects the bulk of the copper from further corrosion. However, in conductive traces, where the copper is formed from a layer having very thin width and thickness, “surface” corrosion can potentially break the conductive pathway or degrade performance of the conductive trace. Copper also reacts with sulfides, such as hydrogen sulfide, to form various copper sulfides on the surface of the copper. In reacting with sulfides, the copper corrodes, as is seen when copper is exposed to air containing sulfur compounds. Oxygen-containing ammonia solutions also react with copper to produce water-soluble complexes, as do oxygen and hydrochloric acid to form copper chlorides and acidified hydrogen peroxide to form copper (II) salts. Copper (II) chloride and copper react to form copper (I) chloride. Therefore there is a need to protect the copper traces from corrosion.
Conductive traces are typically formed by either subtractive or additive processes. Generally, in a subtractive process, copper is coated on a substrate and unwanted portions are removed to leave thin traces of copper. One problem with conventional subtractive processes is that they produce unwanted waste. Subtractive production techniques often begin with copper applied to one or both sides of a substrate. The trace is formed by etching away the unwanted copper from the substrate, leaving behind thin conductive copper traces on the substrate. The etching process typically utilizes ammonium persulfate or ferric chloride. The chemicals and removed unwanted copper is corrosive and toxic and produces environmental concerns and excess waste. Additionally, etching times are comparatively long. Further, as the etchant is repetitively used, copper saturates the chemical etchant making it progressively less effective in subsequently removing copper.
Generally, in an additive process to form traces, copper is formed on a substrate only in areas that form a trace. One problem associated with forming conductive traces by conventional additive processes is that the processes require multiple steps involving various equipment and machines. In a typical additive process, a substrate is imaged with a photosensitive film to produce an exposed pattern. The exposed pattern is subjected to a chemical bath to make the pattern capable of bonding with metal ions. The sensitized areas are then plated with copper to form the traces. The mask is then stripped from the substrate leaving only the copper traces.
Problems associated with both additive and subtractive conventional production techniques is that the copper traces, once formed, need to be protected against corrosion and shorting of the traces due to condensation. The traces are treated with a protective coating after being formed to protect against corrosion. This procedure requires an additional step that involves additional time, money, and equipment. Another problem with conventional production techniques is that the copper traces formed on a substrate exhibit only one color. That is, if the trace is applied to glass or a transparent plastic, the copper color of the trace is visually apparent from one or both sides of the substrate. Another problem associated with conventional techniques is that the substrates must be treated in order for the copper to appropriately bond to the surface. This again requires an additional step necessitating an investment of time and money.
One further weakness in conventional plating techniques is that the thin copper traces are subject to wear and abrasion and the conductive path is easily broken. When abraded to a point where conductivity is broken, the conductive trace becomes inoperable for its intended use.